For many purposes, especially in agriculture, it is desirable to be able to apply spray of a consistent mean droplet size and spray fan angle or cone angle, and evenly distributed, over a range of volumetric spray rates. For agricultural spray nozzles, spray output is classified by the British Crop Protection Council as Fine, Medium or Coarse based on mean droplet size, and chemical manufacturers may specify one of these classifications for applying their product.
However, a typical mass-produced flat-fan-spray nozzle cannot achieve the desired consistency if the flow rate deviates by more than 20% above or below its rated output, and even sophisticated nozzles cannot cope well with deviations beyond 30%.
A requirement for a wider range of flow rates can arise for example in so-called precision farming, that is, applying matter (such as fertiliser or weedkiller) to the ground in amounts varying from spot to spot according to the requirements ascertained from a previous mapping of the ground. The mapping and the application of matter can be made to coincide on the ground using global-positioning-satellite techniques.